One method to control intake manifold pressure of an internal combustion having a variable event valvetrain is described in U.S. Pat. No. 6,439,175. This patent describes methods to control an electronic throttle and timing of electrically actuated valves during periods of engine operation when intake manifold pressure reduction is requested. In one example, the throttle plate position of an electronic throttle located up stream of the intake manifold is adjusted so that a requested pressure is created in the intake manifold. Intake valve timing can then adjusted in accordance with intake pressure so that the desired cylinder air amount is inducted. Intake manifold pressure reduction requests may originate from the vehicle brake system, fuel purge system, or the crankcase ventilation system. The intake pressure is scheduled as a function of engine coolant temperature and whether or not a pressure reduction request is present.
The above-mentioned method can also have several disadvantages. Specifically, the method does not recognize that it may be beneficial to control manifold pressure as engine speed varies, it merely controls manifold pressure based on peripheral system pressure reduction requests and engine coolant temperature. As a result, the engine efficiency may not be as high as desired, at least during some operating conditions. Further, the method may allow engine emissions to increase during some operating conditions. In particular, operating an engine at certain conditions can change the flow dynamics through the engine, and thereby alter the ability of the engine to capture residuals (i.e., combusted exhaust gases used to produce internal exhaust gas recirculation (EGR)) in the cylinder or to control flow from the intake valve to the exhaust valve during periods of valve overlap. If fuel flows from the intake manifold to the exhaust manifold or if some exhaust residuals are not allowed back into the cylinder then emissions of hydrocarbons and NOx can increase. Further, fuel economy may decrease. The above-mentioned method may not be capable of recognizing or compensating for these conditions.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method of controlling intake manifold pressure that offers substantial improvements.